Proton exchange membrane (PEM) fuel cells can be operated in an open mode or a dead-end mode.
In the open mode, excess water is discharged from the fuel cell by continuous convective forces provided by the flow of excess gases. Consequently, gas utilization rates in such systems are typically less than 100 percent (%) and thus fuel and oxidant are supplied at a higher rate to an open mode fuel cell than is required for a complete electrochemical reaction. To increase gas efficiency, unreacted fuel and/or oxidant is usually re-circulated back into the fuel cell using a blowing or pumping system, resulting in a higher pressure drop in the fuel cell and additional power consumption.
In the dead-end mode, at least one of the outlet ports of a fuel cell is blocked. Advantageously, lower power consumption and a higher fuel/oxidant utilization rate are achievable with a fuel cell operating in the dead-end mode. A disadvantage though is that a periodic gas purge is required of conventional dead-end fuel cell systems to prevent flooding. Conventional dead-end fuel cell systems are provided with a purge valve to remove excess water and this complicates the control system. Furthermore, unreacted fuel/oxidant is lost during the purge process and this reduces the gas utilization efficiency.
It is therefore desirable to have a dead end fuel cell system that is able to prevent flooding without purging.